Integrated circuit switches used in integrated circuits can be formed from solid state structures (e.g., transistors) or passive wires (MEMS). MEMS switches are typically employed because of their almost ideal isolation, which is a critical requirement for wireless radio applications where they are used for mode switching of power amplifiers (PAs) and their low insertion loss (i.e., resistance). MEMS switches can be used in a variety of applications, primarily analog and mixed signal applications. One such example is cellular telephone chips containing a power amplifier (PA) and circuitry tuned for each broadcast mode. Integrated switches on the chip would connect the PA to the appropriate circuitry so that one PA per mode is not required.
Depending on the particular application and engineering criteria, MEMS structures can come in many different forms. For example, MEMS can be realized in the form of a cantilever structure. In the cantilever structure, a cantilever arm (suspended electrode) is pulled toward a fixed electrode by application of a voltage. The voltage required to pull the suspended electrode to the fixed electrode by electrostatic force is called pull-in voltage, which is dependent on several parameters including the length of the suspended electrode, spacing or gap between the suspended and fixed electrodes, and spring constant of the suspended electrode, which is a function of the materials and their thickness. An alternative MEMS structure is a bridge, which has both ends fixed to the wafer.
MEMS can be manufactured in a number of ways using a number of different tools. In general, though, the methodologies and tools are used to form small structures with dimensions in the micrometer scale with switch dimensions of approximately 5 microns thick, 50 microns wide, and 200 microns long. Also, many of the methodologies, i.e., technologies, employed to manufacture MEMS have been adopted from integrated circuit (IC) technology. For example, almost all MEMS are built on wafers and are realized in thin films of materials patterned by photolithographic processes on the top of the wafer. In particular, the fabrication of MEMS uses three basic building blocks: (i) deposition of thin films of material on a substrate, (ii) applying a patterned mask on top of the films by photolithographic imaging, and (iii) etching the films selectively to the mask.
For example, in MEMS cantilever type switches the fixed electrodes and suspended electrode are typically manufactured using a series of conventional photolithographic, etching and deposition processes. In one example, before the suspended electrode is formed, a layer of sacrificial material, e.g., silicon, is deposited under the MEMS structure, to form a cavity, the MEMS suspended electrode is formed, and silicon is deposited over the MEMS structure to form a cavity. The cavity over the MEM is used to support the formation of a cap or lid, e.g., SiO2/SiN dome, to seal the MEMS structure. However, this poses several shortcomings. For example, although silicon is an excellent sacrificial material, PVD silicon has poor conformality and slow throughput; whereas, CVD silicon has oxygen or other impurities which prevent venting and also can have poor adhesion. The use of silicon material can also result in undercuts, lid pinning or rubbing, as well as the need for increased contact forces.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.